It is known to pack tea or other goods into bags which are brewed up with hot water for use. The second ply of heatsealable synthetic material/heatsealable synthetic fibers serves to close the bag by heatsealing on high-speed packing machines. As synthetic material/synthetic fibers it is possible to use, for example, polypropylene, polyethylene, interpolymers of polyvinyl chloride and vinyl acetate and also various polyesters.
These bag materials can be produced in known manner by a wet-laid process on a paper machine, by a dry-laid process on a webbing machine or by a melt-blown process by laydown of polymeric fibers on a support layer.
The basis weight of the first ply of material is generally in the range of 8-40 g/m2 and preferably in the range of 10-20 g/m2, and the basis weight of the second polymeric fibrous ply is in the range of 1-15 g/m2 and preferably in the range of 1.5-10 g/m2.
EP-A-0 380 127 describes a heatsealable paper for tea bags which has a basis weight of 10-15 g/m2 and which for heatsealing has been provided with polymers such as polypropylene and/or polyethylene and/or an inter-polymer of vinyl chloride and vinyl acetate.
EP-A-0 656 224 describes a filter material especially for producing tea bags and coffee bags or filters having a basis weight between 8 and 40 g/m2, wherein the heatsealable ply consists of polymeric fibers, preferably of polypropylene or polyethylene, which is laid down in the hot state onto the first ply, which consists of natural fibers.
Prior art heatsealable filter materials have the disadvantage that the seal seams formed by heatsealing are not strong enough for a multiplicity of applications. In some instances, they reopen while still on the packing machine or else later on contact with boiling water. One reason for this is that the production of filled, heatsealed tea bags on high-speed packing machines occurs at a cycle time of about 1,000 bags per minute.
Heatsealing rolls generally seal the bag at a temperature of 150-230° C. in a cycle time of less than 0.1 second. At the end of these short cycle times, the executed seal seams have to be firmly closed in order that no contents may escape. This is frequently not the case, however.
Another reason for the inadequate strength of seal seams is frequently that the shape of tea bags is adapted to consumer demands, and at present it is a round embodiment which is attracting more and more interest. This round tea bag has for the same fill level a seal seam width which is roughly half that of the conventional rectangular bag, increasing the risk of opening.
Attempts to increase seal seam strength by increasing the fraction of available heatseal fibers to above 50% based on the total basis weight of the filter material, occasion a reduction in mechanical properties, such as filter material strength and stiffness. Furthermore, increasing the fraction of heatsealable polymeric fibers increases the barrier properties of the filter material, leading to a distinct reduction in tea 5 extraction. Since these properties, however, are necessary for a smooth packing operation, this approach does not lead to a filter material having excellent heatsealability and good seal seam strength in the dry and in the wet state.
It is an object of the present invention to provide a filter material having excellent heatsealability and good seal seam strength in the dry and in the wet state that overcomes the aforementioned disadvantages of the prior art.
It is another object of the present invention to provide a process for producing such filter materials.